The difference between someone who makes 15k a year and 500k a year is primarily hard work, personal responsibility, and merit. The difference between the guy who makes 500k a year and 500 million a year, however, is primarily luck, having a rich, well-connected family that can mentor you effectively from birth, and taking advantage of the many opportunities only afforded to the super-rich. (This is just a general rule, of course there's exceptions.) I believe this because I don't think the average 500k guy is lazy, dumb or not already obsessed with his career and doing everything he can to get ahead in life.
So, making it to the ranks of the super-rich is like winning the lottery. You have to be smart and obsessed, but 99% of those people only make it to the 500k - 1 million range. What's missing? Luck and connections. Sorry, but if we took 1% more from those people (which will have zero impact on their standard of living) and gave it to the people who win the reverse lottery in life (being born into an abusive family, getting schizophrenia in your 20s), so that they don't have to live like animals for the rest of their lives despite trying their best with the hand they were dealt, I'd sleep like a baby.
Plus, the supply-side argument--i.e., the super-rich invest, take away from them and the economy suffers for everyone--is stupid. I could just as easily say poor people spend most of their income, give to them and the economy prospers for everyone. Without hard numbers and actual research, anyone can make up a superficially-plausible argument to support their view.